1. Field of the Invention
The present invention relates generally to inkjet printing and, more particularly, to a rotary printhead disc in a rotary inkjet imaging apparatus.
2. Description of the Related Art
An imaging apparatus such as an inkjet printer, forms an image on a page of print media by ejecting ink from a plurality of ink jetting nozzles of an inkjet printhead to form a pattern of ink dots on the page. Such an inkjet printer typically includes a reciprocating printhead carrier that transports one or more inkjet printheads across the page along a bi-directional scanning path defining a print zone of the printer.
Market pressures continue to drive improvements in print speed and print quality. There are well-known barriers inhibiting the achievement of rapid printing of high quality inkjet images when using conventional swathing inkjet printheads. For instance, a significant fraction of total print time is dedicated to acceleration and deceleration of the printhead between printing passes. The necessity to camouflage systematic dot placement errors caused by printhead and motion control errors typically requires that each row of pixels (print grid cells) be printed with multiple nozzles. This in turn requires passing the printhead over a given row of pixels several times and advancing the page of media by a fraction of a swath height between passes. Total print time increases as the number of passes increases. Conventional swathing inkjet printers are susceptible to print quality problems due to the reciprocating motion. For example, printhead carrier vibrations during and after each acceleration induce dot placement errors. Reversal of the print direction in successive passes causes noticeable changes in dot shape so that two color tables are necessary for bi-directional printing. Ink dry time is comparable to the printhead turnaround time so dots from the immediately previous pass are wet at one end of the print swath and dry at the other end, a circumstance that can cause undesirable color effects. The paper feed mechanism is susceptible to advance distance variations both across the page width and between successive advances. The importance of paper feed inaccuracies has only increased as print speed requirements have driven manufacturers toward larger silicon printhead chips. All these errors contribute to color, grain, and banding defects.
An ideal multi-pass printhead would be one that achieves the following goals: (1) minimize non-printing time; (2) do multi-pass printing in the same time it takes to do single-pass printing, and (3) eliminate the motion control difficulties inherent in reciprocating printhead motion and incremental paper feed during printing. An ideal device that achieves these goals would be capable of a significant performance improvement compared to a conventional reciprocating printer.
One recent approach as an alternative to the conventional multi-pass swathing printhead is the provision of an inkjet drum printing arrangement in which a printhead moves linearly parallel to the axis of drum rotation as the drum rotates, causing image placement in a helical pattern on the drum after which, once the entire image is on the drum, a page of print media is rolled against the drum under pressure, causing transfer of the image to the page. This arrangement is disclosed in U.S. Pat. No. 7,052,125 assigned to the assignee of the present invention. While this alternative approach might be judged as a step in the direction toward achieving multi-pass printing as the printhead traverses the length of the drum once per page, it is more complicated than desired in view of the two-stage printing process.
Another alternative approach recently introduced in the marketplace is the provision of an array of multiple silicon printhead chips arranged to print a complete page width at once. In such a printer the print medium is fed continuously past the stationary page-wide printhead. High print speed can be achieved at the cost of a large number of silicon chips. This printing arrangement does not lend itself to multi-pass printing so it is susceptible to noticeable banding defects if one or more nozzles fail to jet properly.
Thus, there is still a need for an improved high-speed printing system.